Cryptosporidium parvum is an enteric protozoa which primarily infects the gastrointestinal tract. Infection in immunodeficient individuals, particularly with AIDS, can lead to chronic life- threatening diarrhea against which there is no consistently effective therapy. The purpose of this NCDDG program is to closely examine: i) the molecular structures and sequence of events associated with parasite attachment and cell invasion (Project 1); ii) the nature of the host cell injury that ensues (Project 2); and iii) whether, and how, exogenous monoclonal secretory IgA may arrest this process (Project 3). Project 1 will: i) identify, characterize and clone the sporozoite surface lectins responsible for attachment and invasion and determine their biological role, and ii) identify and characterize the host cell surface receptor responsible for attachment. The project will begin by characterizing a sporozoite surface lectin with hemagglutinating activity which has already been isolated. Identification of the carbohydrate binding proteins and their ligands will provide a basis for developing strategies designed to interfere with parasite attachment (eg, recombinant receptor, receptor analogues, exogenous lectins). Project 2 will define the nature of the cellular injury induced by Cryptosporidium infection by defining the temporal course of alterations in epithelial transport function, and to macromolecular permeability and determine the sites of leakage. Remodelling of the cytoskeleton induced by Cryptosporidium infection will also be characterized, as cytoskeletal changes may play a role in both parasite entry and secretion in diarrhea. These studies will establish the underlying mechanism of diarrhea, and will provide a rational basis for intervention in the short term. Project 3 will determine the therapeutic role of secretory monoclonal IgA released systemically or administered orally on the course of infection in mice. Production and use of human specific monoclonal IgA should then be considered as a potential therapy. Project 3 will also provide administrative and service components to the Program. Oocyst production, and animal assays requiring challenge with Cryptosporidium for the 3 projects will be performed at TUSVM as part of Project 3.